Direct positive photoconductography



Jan. 12, 1965 D. PEARLMAN DIRECT POSITIVE PHOTOCONDUCTOGRAPHY Filed May 15, 1961 LAYER CONTAIN/1V6 REDUC/BLE MATERIAL .5 47 I mm P J 5 6 REAGENT TO FORM COLOR m P w D/AZON/UM SALT w m M 0 M m 0 ATTORNEYS United States Patent .DHKECT PQSITIVE PilDTOCUNDUCTDGPHY Donaid Pearinian, Rochester, N .Y., assignor to Eastman Kodak Company, Rochester, N.Y., afcorporation of New 'Eersey Filed May 15, 1961,"Ser. No. 109,977

8 Claims. (Cl. 204-48) Serial No. 45,941, Raymond F. Reithel, Photoconv ductolithography Employing Nickel Salts, now abandoned. v

Serial No. 45,942, Raymond F. Reithel, Photoconductolithography Employing Magnesium Salts, now U.S. Patent 3,053,179.

Serial No. 45,943, Raymond E. Reithel, Photoconductography Employing Spongy Hydroxide images, now abandoned.

Serial No. 45,944, Raymond F. Reithel, Method for Making Transfer Prints Using a Photoconductographic Process.

Serial No. 45,945, Raymond F. Reithel, Photoconductography Employing Manganese Compounds.

Serial No. 45,946, Raymond F. Reithel, Photoconductography Employing Molybdenum or Ferrous Oxide, now abandoned.

Serial No. 45,947, Raymond F. Reithel, Photoconductography Employing Cobaltous or Nickelous Hydroxide, now abandoned.

Serial No. 45,948, Donald R. Eastman, Electrophotolithography.

Serial No." 45,949, Donald R. Eastman, Photoconductolithography Employing Hydrophobic Images.

Serial No.'45,950, Donald R. Eastman and Raymond F. Reithel, Photoconductography. Employing Electrolytic I Images to Harden or Soften Films, now US. Patent Serial No. 45,951, Donald-R; Eastman and Raymond v F. Reithel, Photoconductography Employing Absorbed Metal Ions, now abandoned.

, Serial No. 45,952, Donald-R. Eastman and Raymond F. Reithel, Photoc'onductography Employing Spongy.

' Images Containing Gelatin Hardeners, now US. Patent SeriaI No; 45,953, John l-Sagura, Photoconductography Employing Alkaline Dye Formationf now Pat- (int I H I v v Serial No. 45,954, John J 'Sagura'and "James A. V an Allen, Photoconductography Employing Quaternary Salts.

Serial- N o. 45,955, Franz and Nelson R. Nail, I

Uniform. Photoconductographic Sheets, now abandoned. 7

Serial No. 45,956,"Franz Urbach and Nelson Nail,

High Contrast Photoconductographic Recording, now

abandonedm g 9 Serial No. 45,957, Nicholas L. Weeks,-- Photocon ductogr'aphy'lnvolving Transfer of Gelatin.

Serial No. 45,958, Donald R. Eastma'n, .Photoconent 3,095,808. I

ductolithography- Employing Rubeanates," now US. Pat- Recording on Flexible non-color forming.

c fas ened Jan. 12 was ,Serial No. 45,959, DonaldR. Eastman and'Rayrnond E.' Reithel, Electrolytic Recording With Organic Polymersfnow US. Patent 3,106,155.

. 'Serial No. 46,034, Franz Urbach and Donald Pearl rnan, Electrolytic Recording, now abandoned.

Cross reference is also made to the following three applications filed October 25, 1960:

Serial No. 64,901, Simultaneous Processing in Photo- .conductography,. FranzUrbach. Serial No. 64,902, Solid Recording System for Photo-.

conductography, Nelson R. Nail.

Serial No. 64,903, A Two-StepReversal Processing Method in Photoconductography, Franz Urbach.

Electrolytic facsimile systems are well known. Electrolytic photoconductography is also known and is de: scribed in detail in British 188,030 Von Bronk and British 464,112 Goldmann, modifications being described in British 789,309 Berchtold and Belgium 561,403 Johnson et al. The present invention is primarily used in photoc0nductographic processes because its advantages (direct positive production) are greatest there. However, the invention is also useful in facsimile or any electrical recording system.

The object of the present invention is to produce a direct positive image in a simple, efficient manner and to produce such an image that is stable and has highresolution.

According to the present invention an image in the form of variations in electrical conductivity is established in a photoconductive layer such as selenium or zinc oxide in resin. in the case of selenium the image must be developed during exposure and in the case of zinc oxide photoconductors, the image may be developed either during or immediately following exposure during the period of persistence of the image. I

This image of electrical conduction is converted to a visible image by a process which involves electrolytic reduction. One characteristic of the present invention whichdistinguishes it from former photoconductographic processes is the fact that the electrolytic reduction is employed to render a material relatively non-reactive and The material which has not been so reduced is then reacted with a second reagent to form a coloring material pigment or'dye. Since the coloring material is formed in the non-conductive unexposed areas of the image, the resulting print is a direct positive.

Thus the essential feature of the invention involves a color forming reaction of two or more components, one 7 of which is reducible by electrolysis and when in its reduced state does not react with the other'component or components to form'the colorant. When the reducible component is relatively insoluble and becomes soluble in its reduced state, as is sometimes thecase, the reduc-., tion product preferably may be removed by washing .be-'

fore the color forming step Before or after exposure to establish the conductivity image, the exposed; photoconductor is, or is placed, in

- "contact with an electrically conductive layer which con'-.

tains the reducible color forming component. The layer, in some cases, is on a;suppor t separate from the photoconductor and in other cases in whichthe photocond'uctor' itself is to become the fi nal'suppo'r t for theprint, a

a suitable coating containing the reducible component is applied directlyto the photoconductor. 7 While the reduciole component :is .in contact with theexposed photoconductor, an electrical potential is: applied which causes current to flowth'rough the exposed areas of the photoconductor and theadjacentareas" of the reducible layer.

These are referred to as the areas-of higher current ,density. f

Thus the invention requires currentto be passed thro'ugh the' reducible layer and --d'uring the flow of ing areactive positionpara to the hydroxyl group. 7

electric current, the reducible component is reduced at the'cathode and is thus rendered incapable of later reacting with the-other color forming components. In those embodiments in which the reducible layer is on. a separate support, it ,is removed from the photoconductor at this stage 'of the process. Whether the reducible component is on a separate support or on the photoconductor, the next step of the process consists. of treating it with the other color forming component or components. These 'react the unreduced material, in the unexposed areas, to form a dye or pigment. The endresult is a direct positive' print. 1 r

In certain color forming processes, both components are reducible to a non-reactive state, in which case either of the components may be the one subjected to electrolytic reduction.

For clarity the component which is to be subjected to 'aieaasr In color photography certain:couplers are preferable for the color they produce, or for variou's'other reasons 'not involved in straight recording. Hence the-present in- I.

vention can utilize many of the couplers which are im-' practical for color photography as well as those which are actually used.

present invention can also be, operated using inorganic systems. For example manganate ions in thefform of a coating of manganese .dioxidecan be reduced to the manganous, ion. In the oxidized form themanganate ion the imagewise distributed electrolytic action is referred to I ly to a hydrozine derivative and, as is known, the azo dyes cannot be prepared by direct reaction of such hydrazine with a color coupler. The reduction of a diazoniumsalt to ahydrazine is Well known and a description can be found in Technique of Organic Chemistry, A. Weissberger,

vol. 2, second edition, page 518.

As a second example, the reducible component may be any of the typical couplers used in color photography which react with oxidized developers'to form an indamine, indophenol, indoaniline or azomethine dyes. Moreover, this process is an example of the systems in which either component may be the electrically treated one. That is,

the reducible component may alternatively be the oxidized developerwhich when not reduced will react with a typical coupler to form a dye, but which in its reduced or un-,

oxidized state is unreactive. v

' Thereare many forms of this color forming reaction through which the exposure is made.

will react with a leuco dye and oxidize it .to a color whereas the manganous ion is-incapable of such reaction. In

this embodiment of the invention a layer containing manganese dioxide-' is subjected to electric current which reduces it to be manganous form and this layer is then placed in contact with a-layer containing a leuco dye.

.Theorientation of the print depends on Whether it is on the photoconductor itself or an 'a separate support One is the mirror image of the other. In projection printing, this question of orientation of the image is taken care of by properly orienting the original positive transparency In those embodiments in which the photoconductor is exposed by contact with a positive image, the reducible; layer is'preferablyt on a separate support since the resulting orientationof the image is then identical with that of the original "positive 7 being copied.

Which maybe used; Typical couplers embrace a wide variety of chemical structures. be classified in three groups: (1) Openchain methylene compounds, (2) cyclic-methylene compounds, and (3) methine compounds. See The Chemistry of Dye-Form These conveniently can ing Development byP. W. Vittum and A. Weissberger,

Journal Photographic'Science, vol. 2, 1954, pagesf8'l-to Theopen chain methylene-compounds are, repre- 96. sented by the type formula XCH Y in which Xand Y are the same or different electron-attracting groups which render the CH groups more or less strongly acidic. The reactivity of the couplers toward oxidized developer, as well as the color of the dye formed, varies widely with-the different combinations of the X- and Y- groups. There 'is a chart on-"page 85 of the Vittum open-chain methylene couplers are reducible groups} Re- The commoner forms of open-chain methylene compounds areIa-acylacetanilides and oi-acylaminoacetanilides.

.i Similarly, Table II,- ;page 87, of the paper presents cyclic methylene couplers in'which the methylene group is activated byeasilyreduciblie groups. Reduction 'of'oneof 'layer move synchronously The invention and its advantages, will be fully understood from the following examples and from the description when read in connection with the accompanying drawing in which: 1 FIG. 1 schematically illustrates a process according to a preferred embodiment a of the invention.

FIG. 2 similarly illustrates'an alternative embodiment in which the final image is formed on the photoconductor.

FIG. 3 shows the process involving development during exposure of the photoconductor rather than subsequent thereto as shown in FIG. 2. V r

In FIG. 1 the image of a positive transparency 1G illuminated by alarnp 11 is focused by a lens 12 on a photoconductivelayer 15 of zinc. oxide in suitable, resin binder carried on an electrically conducting support 16 Which may be highly conducting paper orwhich preferably includes a metal foil layer. The transparency 10 is moved to the left as indicated by'thea rro w 17 while the photoconductive layer 15 is moved to the right as indicated by the arrow 18 so that the image and photoconductive to expose the layer part at a time. 7

- in contact with a layer 25 containing a reducible material 'cluctionof either X' or Ydestroys the coupling ability.

andan ionic salt to be conducting, carried on a support 25. Qontact. between the two layers is assured ,by' pressure rollers 20 and 21 which vact as electrodes-.. The electric current is supplied by a source indicated at 22. 7 A DC; 7

source is shown inthis figure but since the'zinc oxide, electrolyte, interface acts as a rectifiergan' AC. source could be, used'as wellfand is often more convenient.

The current flowing through the exposed areas of the 'photoconductor andthegadjacent areas of the layer 25' reduces the reducible cornponent'in its layers so that the layerzs then contains-"areas with the reduced moiety and areas withunreducedmaterial corresponding to the unwith'water. as illustrated n21 which removes the rethegroupsdestroys thecoupling ability; The commoner forms ofv cyclic or closed 'methylenes are the S-pyrazolonesv whichtar'e reactive in the 4-position, i.e., which havea reactive methylene group adjacent to a carbonyl group. Finally Table .III' on'pa'ge 90 presents methine couplers'. Reduction of'the'methine flinkagealso destroys the coupling ability. These include phenoliccompounds contain- I duced material ing-those cases in which thelatter'is soluble and. the unreducedma'terial is not soluble. This is the usual situation and'renioval ofthe reducedmaterial leads to higher uality final prints whichflare stable and not desirable to include such" a step in actual; practice ofthe invention. 'Thelayer'ZS isthen moviedincontact with The exposed photoconductive layerlS is then pressed the roller 30 which applies the other reagent or reagents 31 which reagents react with the unr'educed material to form the coloranta'This reagent 31 'does not have any eifect' on the reduced area even if the reduced material has not been fully removed ,by the wash 27. Accordingly I a source '36 is applied. Current flowing through theexposed area of the layer reducesjthe diaz'onium salt to a hydrazine producing a layer 37 which is essentially a photoconductor having both an oxidized and a reduced form of a material, namely a diazonium and a hydrazine, distributed imagewise onthe surface thereof. The layer 37 is then washed with water 38 and is thentreated by means of'a brush 39 witha coupler which forms a dye 'with the unreduced diazonium thereby forming a positive print'40. A

Both FIGS. 1 and 2 employ zinc 'oxidein resin as the 'photoconductor and rely on the persistence of the conducting images therein. parency 43 illuminated by a lampdll is focused by "a lens "dz to forrna complete image over the area of a photoconductive layer 45 which in this case may be either In FIG. 3, on the other hand, a transselenium or zinc oxide in resin.- The photoccmductive 4 layer is carried 'on a transparent conductive support 45 such as glass having a thin layer of metal or metal oxide on the surface thereof adjacent to "the layer 45. The layer 45 is placed in contact with an image receiving sheet '48 havinga reducible salt contained in a surface layerdia In FIG. 2 thelayer 15 before orsium chloride, 1.5 g. glycerin and 0.3 cc. formalin was coated to a wetthickness of approximately 0.007 inch on a sheet of baryta coated paper.

of potassium chloride and then was pressed into contact with a layer of zinc oxide in resin coated on a metal foilpaper laminatewhich had been exposed for 600 ft. candle seconds to a positive image. With the zinc oxide layer as cathode, 60 voits DC. potential was applied across the electrode contacting opposite sides of the sandwich and the current-was allowed to fiow for'S seconds. The paper containing gelatin and the diazonium salt was then removed from the photoconductor. The surface was bathed in an 0.4% :50 alcohol-water solution of l-hydroxy-N-octadecyl-4-sulfo-2-naphth-amide (coupler) at a pH of about 8-10. The coupler combines with the unreduced diazonium salt' but does not form any im'age in the area where the hydrazine was produced; A high reso lution, good density print was obtained.

Example 3 I v v A gelatin layer approximately 0.005 to 0.007 inch Wet I thickness was'pr'eparedfrom50 g. of a 5% solutiomof paper and dried. The surface thereof was moistened with gelatin Which contained 0.2" g. 2,2'.-dirnethoxybenzidinetetrazonium chloridezinc chloride doublesalt, 0.2 gram" potassium chloride, 1.5ggrams glycerin, and 0.3 cc. for'mwalin. This layer was coated on a sheet of b arytaicoa-ted a solution containing 2% by Weight of potassium chloride and it was then pressed into contact with a layer of zinc -oxide in resin which had been coated on a metal foilpaper laminate andwhich had been exposedfor 600 ft.

' candle seconds toa positive image. Sixty volts DC.

potential was applied across the electrode contacting oppositesideso'f the sandwich, the zinc oxide layer being i the cathode, and current was allowed ,toflow for 5-10 Current is passed from an A.C. or DC; source indicated schematically at 50 between the electrode '46 and the con-- "ducting 'support48. The current flowing through the layer 47 reduces thev reducible salt and this reduced salt then may be washed away by water 50. The unreduced moiety remaining in the layer 47 is then treated with a suitable reactant by a brush 51 to form a positive print in the layer '47.

' Example 1 A gelatin layer containing a solution of 50 g. water, 2.5 g. of gelatin and 0.2 g. of p-N-morpholinobenzenediazonium zinc chloride, 02. g. potassium chloride, 1.5 cc.

glycerol and 0.3 gfiformalin was coated on a paper support and dried. The surface thereof was brushed lightly with a 2-5% solution of sodium chloride to moisten the surface andrender ithighly conducting. I The layer was then pressedin Contact with a zincqoxide in resin layer coated on a metal foil paperlaminate which had been and thereby forms a' dye could have been used; The

coupler combines-with thef unreduc'ed diazonium salt but. does not-.form any image in the area where the hydrazine I was produced. A high resolution, good density print was obtainedin this example. 3

A gelatin layer containing a solution of 50 grams water,

Any compound-which I exposed f0r2400 ft. candle seconds to-a halftonepositive ,image; ,70'-volt .D.C. potential was applied across the "electrode contacting opposite sides of the sandwich'fiand seconds, The paper containing the'gelatin -and the tetrazoniu'm salt was removed-from the photoconductor, the surface was bathed in an alcoholic solution 0t l-hydroxy- 3',5'-dicarboxy-N-octadecyl-ZfnaphthaniIide :at a pH:

8.5-9. The coupler combines withthe' unreduced tetrazoniurnsalt but does not form any imagein the area Where Y the material was reduced- A high resolution, good density print'wasobtained in this example.

Example This example was identical with the above Example 3 exceptthat in place of the tetrazonium reducible salt, pnitrosodimethylaniline was used, and for the coupler in Example 5 I A gelatin layer cont-aininga solution or" 50igranis of water, 2.5 grams gelatin, 0.2 gram potassium chloride, 1.5

{grams glycerin,,0.3 cc. formalin, and'0.2 g'raml-p-suljphophenyl-3-methyl-5-pyrazolone was-coated to a wet face thereof was moistened with asolution containing '2% by weight of potassium chloride and then was rolled "into contact with a layer of Zinc oxide in resin which was coated on a'metal foil-paper laminate and then exposed for/L00 ,ft.-.candle' seconds to a positiveimagej With the zinc oxide layeraslthe cathode,wa-'D.C. potential of 60fvolts was applied for 5 seconds across the electrode.

.The' paper containing the gelatin. and the'pyrazolone was i then removed from the zincoxide surface and moistened in a solution ofp-(N-morpholino)benienediazonium zinc chloride, zinc chloride double salt; which was at a pH of 8-"10. The'diazoniu'm salt combinesfwith the unredu'ced 2.5 g.'.gelatin, 0.2 'g'.'of 2;methoxy-4-pentadecylbenzene' "diazonium chloride, zincjchloridedouble salt, '0.2 g. potascoupler but doesncit-forna any image in the areas where the coupler was reduced. A'high resolution, good density, postive print was obtained inthis example,

7 The surface thereof was moistened with a solution containing 2% by weight thickness of 0.005 inch on gelatin-siiedfpaper. The sun.

sodium chloride.

gelatin layer approxirnately 0005 to 0.01

. E Ex mples Fifty grams of a 5% solution of gelatin containing 0.2

face thereof was moistened with a solution containing,

2% byweight ofpotasiuni chloride.

coated on a inetal foil-paperlaminate which had been exposed for 600 foot candleseconds to a positive image. 'Sixty vo1ts' A.C. potential was applied across the electrodes contacting the opposite sidesof the sandwich, and 'currentwas allowed to flow for l seconds. The paper containing the gelatin and the'pyrazolone was removed from the photoconductor. The surface was rinsed briefly I with water, then bathed with a solution of 4-amino-3- 'methyl-N,N-diethyl-aniline, freshly oxidized by the addition of a small amount of potassium persulfate and at 'a pH of 9-11. The unreduced pyrazolone combines with the. oxidized developer but does not form an image in: i

the area-where the pyrazolone was reduced. A good density, high quality print is obtained in this example.

Example 7 Then thisv was pressed into contact with a layer of zinc oxide in'resin A layerapproximately 0.005 to 0.010 inch wet thiclt- ,ness of gelatin coated on paper from a solution of 50 gramsof 5% gelatin, containing 0.2 gram acetoacetanilide, 0.2.gram sodium chloride, 1.5 grams glycerine, 0.4

cc. of formalin was allowed to dry. The surface was moistenedzwith asolution containing 2% by weight of was applied across theelectrode contacting opposite sides of the sandwich and current was allowed to flow forlO seconds. The, zinc oxide 'layerwas the cathode. The paper thenwas removed from the photoconductor and the surface was vbathed in-a water solution of a few percent of 4-N-ethyl-N-methylsulfamido-ethyl-3-rnethlyaniline sulfate which had been freshly oxidized with a small amount of potassium persulfate and at a pH of 9-11. The developer combines only with the unreduced coupler and therefore forms a positive image. A high resolution, good density print was obtained in this example.

' Example 8 A layer. on sized paper approximately 0.005 to 0.010 7 inch wet thickness, coated from'a solution of 50 grams of 5%v gelatin, containing 0.2 gram I hydrOxy-Z-N-{T- methoxy-4 [x"-ch lorosulfony1-3 "-(4 -tert.amylphenoxy) v benzamido]phenyl}-naphthamide, 0.2g. sodium chloride, 1.5 grams glycerin, 0.4 cc. ofv formalin was allowed to dry. The surface was moistened witha solution containl ing 2% byweight of sodium chloride. This was pressed into contact'with a layer of zinc oxide in resin coated on a metal foil-laminate: which hadbeen exposed for 400 ft, candle seconds to a positive image. Sixty volts D.C..

potential was applied across the electrode contacting opf posite sides of the sandwich and current was allowed to flow for 10 seconds. The zinc oxide layer-wasithe cath ode. Then the paper was removed from the photoconductor. and the surface-was'bathed in a-water solutionof a few percent of 4 amino-3methyl-N,N-diethylaniline V which had been freshly oxidized 'with a small amount-of This then was pressed into contact; with a layer of, zinc oxide in resin,'coated on-a metal foil- 'paper laminate-which hadb een exposed for 400 ftcan- .dle seconds to a positive image; Sixty volts DC. potential tained in this example,

thicknesspf a :solution of grams of-5 gelatin, containing0.2 grain l-hydroxy 4-phenylazo@2-(o-chlorophen yl)naphth amide, 0.2 gram sodiumPchloride,l.5i.'grarns glycerin, 0L4 cc.,of formalin and. 0.3. gQN-Butylacetanilide, prepared in a colloid mill was coated on sized paper and was allowed to dry. I The surface was moistened with a solution containing 2% by. weight of sodium chloride. This then was pressed into'contact with a layer of zinc oxide in resin coated on a me tal foil-paper laminate which had been exposed for 400 ft. candleseconddto a positive image. Sixty volts D.C. potential was applied across the electrode contacting opposite sidesof the sandwich and current was allowed to flow for 10 seconds. The zinc oxide layer was the cathode.:- The paper was thenremovedfromithe photoconductor and the surface was bathed in a water solution of'a few per cent of I 4-amino-3-methyl N,N diethylanilinewhich "had been freshly oxidized withgasrnall amount of potassium per- .sulfa'te at a pH 01%941. The developer combines only with the'unreduc'ed coupler 'andtherefore forms a positive image. A hi h resolution, good density print was ob- A gelatin layera'pproximately 0i005to 0i0l0 inch wet thickness of a dispersion prepared inacolloidal r nillfrom a solution of 50 grams of gelatin, containing 0.2 gram l-hydroxy-B. ,5 '-dicarboxy-N-octadecyl-Z-naphthani lide,-'0.2 gram sodium chloride, l .5. grams glycerin, 0.4 cc; of formalin and 0.3 'g. N butylacetanilide was coated on baryta sized paper and was allowed to dry. The Sur}. face was moistenedwith a solution containing 2%. by weight'of sodium chloride. This was athen pressed into" conductorr'and the surface was bathed in a water-solution of a few percent of 4-amino-3-methyl-N,N-diethylaniline oxidized with a small amount of-potassium persulfate at a 'pl-l of 9 11. The developer combines only. with the unreduced coupler'and therefore'forms a positive image. A high resolution, good density'print was obtained in this example. r

' 'Example 11 A gelatin layer'containing a solutio'nfof-SO cc. of 5% gelatin, 1.5 g. glycerol, 0.2 cc. formalin, 0.2 cc.-ofa wetting agent, 5 cc. of 0.3% potassium-permanganate which had :been' added slowly .to the restof the material was high .dispersity and therefore the suspension: has. a light. traw.yellowcolor. The 'dried1layerwas moistened; with a solution containing 2% pota'ssium chlorideand'was then rolled into contact Witha zinc oxiderresin layer which had been coated on a vmetahfoil-paper. laminate? and: which 7 potassium persulfate at, a pH' of 9-11. The developer print was obtained in .this exampleg; x I

0: inch wet i material;

with a wet thickness 050.005 inch. Ifi the pern'ianganate is added slo'wly,j;manganese dioxide is obtained.in very hadbeen exposed for800 ftpcandle seconds to a halftone image: A, DC; potential. of volts. was) applied 1 across the electrode contacting opposite sidesiof theisandwich'and current was allowed tolfiowi for lOQseoonds.

- The layer containing the'rnang'anic'salt was removed from the photoconductor; briefly rinsed in'water; and then bathed "in a 2% solution oflNational Solvat Blue -O leuco dye (Na-tionalAnilin'eCompany) a pH Tof 401 A density.

blue direct positive" image was formed in the areas which. i correspond to theunexposed or unreduced areas or the -Thepr nt had veryrhigh ;reso1ution and good wh iia iaii sl I 1% of ferric sulfate and 1 cc. glycerol was coated onto a sized paper support to a wet thickness of about .003 inch, and allowed to dry. The layer was moistened with a solution of 2% potassium chloride and then rolled into contact with a layer of zinc oxide in resin coated on a metal foil-paper laminate which had been exposed for 400 ft. candle seconds to a positive image. Eighty volts DC. potential was applied across the electrode contacting the opposite sides of the sandwich, the zinc oxide being the cathode, and the current was allowed to flow for 8 seconds. The paper containing the polyvinyl alcohol and ferric sulfate was removed from the photoconductor. I

The surface then was bathed with a 1 to.2% solution of potassium ferrocyanide and a blue positive image was formed in the unexposed or unreduced layers. The image was then rinsed and dried. A good quality print was obtained in this example.

Example 13 This example was identical with Example 12 above except that in place of the potassium ferrocyanide, a 2% solution'of ammonium thiocyanate was used. The ultimate print consisted of a red positive image formed in the unreduced area.

The invention has been described in detail with particular reference to preferred embodiments and illustrative examples thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I'claim:

1. In a photoconductographic process the steps comprising forming in a photoconductive layer an image pattern of variations in electrical conductivity, placing the photoconductive layer in electrical contact with a light tone layer containing a uniformly distributed, substantially colorless, material, which material will react with a reagent to form a dye of a darker tone than the initial tone of said light tone layer and which material is redncible to an non-so-reactive form by the passage of electrical current, passing current through both layers with said photoconductive layer acting as a cathode, whereupon said current is distributed in accordance with said pattern to reduce said material in areas of higher current density, and subsequently treating the light tone layer with said reagent to react with the unreduced moiety in the other areas to form said dye.

2. A photoconductographic process according to claim 1 in which said uniformly distributed material and said reagent are both substantially colorless.

3. A photoconductographic process according to claim 1 in which said uniformly distributed material is a diazonium salt which is reducible by the passage of electric current to a hydrazine and in which the reagent is a conpler which couples with the unreduced diazonium salt to form an azo dye.

. veloper and in which said uniformly distributed material is a photographic coupler which reacts with said oxidized developer to form a dye, the coupler being selected from the group consisting of a-acylacetanilides, ot-acylaminoacetanilides, 5-pyrazolones reactive in the 4-position and phenols reactive in the para-position.

5. A photoconductographic process according to claim 1 in which said reagent is an oxidized photographic developer and in which said uniformly distributed material is a photographic coupler which reacts with said oxidized developer to form a dye, the coupler being selected from the group consisting of open chain methylene compounds, I

cyclomethylene compounds and methine compounds.

6. A photoconductographic process according to claim 1 in which said uniformly distributed material is an oxidized photographic developer which when reduced is unreactive and in which said reagent is a color coupler selected from the group consisting of open chain methylene compounds, cyclomethylene compounds and methine compounds.

7. Aphotoconductographic process according to claim 1 in which said uniformly distributed material is a manganese compound which by the passage of current is reducible to a manganous compound and in which said reagent is a leuco dye which reacts with the manganese compound to form a color but which does not react with a manganous compound.

8. A photoconductographic process according to claim 1 in which said uniformly distributed material is a ferric salt which by the passage of current is reducible to a ferrous salt and in which said reagent is potassium ferrocyanide.

References Cited in the file of this patent UNITED STATES PATENTS 863,761 Schmid Aug. 20, 1907 2,038,486 Glas Apr. 21, 1936 2,837,471 Law et a1 June 3, 1958 3,010,883 Johnson et a1 Nov. 28, 1961 FOREIGN PATENTS 6,887 Great Britain Nov. 3, 1894 of 1894 215,754 Australia June 23, 1958 OTHER REFERENCES Weissberger: Technique of Organic Chemistry, volume 2, 2nd edition, page 518, 1949.

Vittum et al.: The Chemistry of Dye-Forming Development, Journal of Photographic Science, volume 2, 1954, pages 81-96.

Grieg, H. 6.: Proceedings of the I.R.E., The Chemistry of High-Speed Electrolytic Facsimile Recording, October 1948, pages 1224-1234. 

1. IN A PHOTOCONDUCTOGRAPHIC PROCESS THE STEPS COMPRISING FORMING IN A PHOTOCONDUCTIVE LAYER AN IMAGE PATTERN OF VARIATIONS IN ELECTRICAL CONDUCTIVITY, PLACING THE PHOTOCONDUCTIVE LAYER IN ELECTRICAL CONTACT WITH A LIGHT TONE LAYER CONTAINING A UNIFORMLY DISTRIBUTED, SUBSTANTIALLY COLORLESS, MATERIAL, WHICH MATERIAL WILL REACT WITH A REAGENT TO FORM A DYE OF A DARKER TONE THAN THE INITIAL TONE OF SAID LIGHT TONE LYAER AND WHICH MATERIAL IS REDUCIBLE TO AN NON-SO-REACTIVE FORM BY THE PASSAGE OF ELECTRICAL CURRENT, PASSING CURRENT THROUGH BOTH LAYERS ITH SAID PHOTOCONDUCTIVE LAYER ACTING AS A CATHODE, WHEREUPON SAID CURRENT IS DISTRIBUTED IN ACCORDANCE WITH SAID PATTER TO REDUCE SAID MATERIAL IN AREAS OF HIGHER CURRENT DENSITY, AND SUBSEQUENTLY TREATING THE LIGHT TONE LYER WITH SAID REAGENT TO REACT WITH THE UNREDUCED MOIETY IN THE OTHER AREAS TO FORM SAID DYE. 